On 2008-10-01, David Lesher wrote:
"DoN. Nichols" writes:
Yes. If you turn out to need a little more capability to start
the saw, hang another three phase motor on the RPC, start it after the
RPC is started, and it will help to start the saw.
I'm debating in my head if adding a flywheel to a RPC will improve its
ability to start big motors. I can generate arguments both way in my
head, but Machines 450 was 20+ years ago, and while I loved the class,
they say the mind is the first thing to go...
It has been considered here in the past -- but tests seem to say
that it does not help. It might prevent the RPC's idler reversing when
you try to "plug" reverse (that is switch the machine to reverse without
letting it slow to a stop first). Sometimes, when the idler in the RPC
is approximately the same HP rating as the load motor, and the load
motor is driving a lot of inertia, when you suddenly switch to reverse,
the idler motor reverses instead of the load motor -- allowing the
machine to keep on going into the crash which you were trying to avoid. :-)
In one manner of thinking a VFD is just a special UPS.
Without the extra power storage (normally lead-acid batteries)
which allows the UPS to keep providing power after the line power
vanishes. :-) But the output part is similar.
But the VFDs normally use PWM to simulate the output sine wave,
not a true sine wave (which you get out of the better UPS, since some
computer power supplies are allergic to the voltage spikes from a PWM
emulation of a sine wave.)
Years ago, I
worked at a company that made big 3 phase UPS's, as from the 10KW to 10MW
size.
Hmm ... Best Power Systems, by any chance? I really *like*
their USPs, and have three of them running at the moment -- all Ferrups
models.
These were ferroresonant UPS's. In mechanical terms, a FR system is a
transmission with a slip clutch and a big flywheel. You can lock the
output shaft, run it all day and nothing gets bothered. [You may know the
brand "Sola transformer"...]
Yep -- "Sola constant voltage transformer". I've also got a DC
power supply in which the only regulation is that it was built around a
Sola style transformer. Probably a bit more efficient than normal
linear regulators, but less so than a switching regulator.
In EE terms, they are short-circuit proof, and the FR 'tank' stablizes
the output across several cycles of line glitches.
A standard lab test for new designs involved a large 3 phase motor.
I recall it was ~4 ft diameter and 6+ long. No doubt it was surplus
from the steel mill.
You started the inverter module [Bigger units were usually collections
of 10KW modules...] up, and closed the breaker to the motor. Not much
happened; the humming got louder but that was it. If you looked at the
metering, you'd see the module was at 100% output power but so what, the
motor was still stopped.
Then, if you looked oh so closely, you could see it twitch and s l o w l y
turn a little and then more and ever so slowly it could be seen to be gaining speed....
Over 10-30 minutes, the motor would evenually come up to ~850 RPM, and the
current would drop to something normal. The module had passed..
Wow! *That* is impressive.
VFDs commonly start (from a full stop) by starting to output
something like 0.5 Hz (and an appropriately low voltage) and then
sweeping the frequency up to the full 60 Hz (here, or 50 Hz in the UK
and some other places). This allows control of the starting surge which
otherwise is present when a motor is simply just switched on. Most VFDs
have a parameter to control how long the ramp-up time is to keep from
drawing too much current and tripping the current limit error shutdown.
It can range from something like 0.1 second to several seconds.
Enjoy,
DoN.
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